This invention relates to production storage of high-magnesium ammonium polyphosphate solutions.
So-called wet-process phosphoric acid is conventionally produced by digesting phosphate rock with sulfuric acid which reacts to form phosphoric acid of a concentration of about 30% P.sub.2 O.sub.5 by weight, and gypsum which is insoluble in the reaction products. The gypsum is separated from the phosphoric acid by the usual means such as filtration settling or both and the resultant weak acid may be used as an article of commerce. For many important commercial uses, such as, for example, production of liquid fertilizers, the weak acid is concentrated by evaporation to provide phosporic acid containing about 50-70% P.sub.2 O.sub.5.
While the wet-process for phosphoric acid is simple in concept, it is fraught with many technical difficulties, and the resultant phosphoric acid is a highly impure material, containing relatively large amounts of dissolved sulfates and smaller amounts of fluorides, fluosilicates and salts of aluminum, magnesium, iron and other metals. These impurities precipitate and settle out as solids or sludge at a slow rate occurring over an interval of many days and even months. The amount of solids which settle out varies with the purity of the phosphate rock from which the acid is derived and is generally from 0.5 to 10 percent weight of acid.
So-called superphosphoric acid containing at least 66% P.sub.2 O.sub.5 content may be economically prepared by evaporating water from wet-process phosphoric acid. The term superphosphoric acid as used herein is defined as phosphoric acid containing substantial quantities of both ortho- and polyphosphoric acids. These polyphosphoric acids include pyrophosphoric acid and other linear polymers.
Ammonium polyphosphate solutions made from wet process phosphoric acid normally contain as metal impurities iron, aluminum and magnesium. Of these, magnesium occurs in smallest concentration but has the most deleterious effect on the stability of the ammonium polyphosphate solutions. Commercial ammonium polyphosphate solutions for fertilizer use normally are made with 45-65% of their P.sub.2 O.sub.5 content as polyphosphate. It has been found that such solutions containing a Mg/P.sub.2 O.sub.5 weight ratio of 0.0056 stored at ambient temperatures, will precipitate solids containing magnesium in about 4 months. Moreover, such solutions containing larger amounts of magnesium are much less stable, e.g., solutions containing a Mg/P.sub.2 O.sub.5 weight ratio of 0.01 will precipitate solids containing magnesium in about 2 days. For purposes of this invention, the present stable, highmagnesium ammonium polyphosphate solutions are defined as solutions which remain stable for at least 250 days although they have a Mg/P.sub.2 O.sub.5 weight ratio of at least 0.0056.
It is known that iron and aluminum ions present as predominant metal impurities in wet-process phosphoric acid, form gelatinous precipitates that render ammonium salt fertilizer solutions prepared therefrom thixotropic and gelatinous. U.S. Pat. No. 3,044,851 discloses that the formation of gelatinous iron and aluminum precipitates can be prevented by heating the acid to expel the fluoride impurities, and thereafter forming acyclic polyphosphoric acids, including pyrophosphoric acid, in the acid. Moreover, this patent discloses that magnesium requires a far higher concentration of polyphosphates to prevent precipitation than the same amount of other metal impurities and suggests that magnesium can be selectively removed from the dilute wet-process said by passing the acid over a cation exchange resin. Unfortunately, such treatment of the dilute wet-process acid is expensive and time consuming.
It has also been suggested to remove the magnesium impurity by "over ammoniation" of superphosphoric acid, analyzing at least 66% P.sub.2 O.sub.5. The superphosphoric acid is diluted with water and then ammoniated to a N/P.sub.2 O.sub.5 ratio of about 0.4 to precipitate the magnesium as a complex ammonium phosphate. This procedure has the disadvantage that substantial loss of phosphate occurs only a small proportion of the complex ammonium phosphate precipitate is magnesium.
U.S. Pat. No. 3,642,439 relates to a method of separating magnesium from wet-process phosphoric acid characterized in that the magnesium is separated as a readily filterable fluoride-phosphate complex compound containing magnesium and aluminum. The fluoride ion required for formation of the magnesiumcontaining precipitate is preferably formed by hydrolysis of fluroine compounds normally in the crude acid. In the case of insufficient quantities of any of said precipitation components, additions of such components are made to the acid or to the reaction components from which the acid is produced. The wet-process phosphoric acid low in magnesium is especially useful for production of stable ammonium polyphosphate fertilizer solutions.
These prior art methods for separating magnesium from wet-process phosphoric acid have made a substantial contribution to the development of commercial liquid fertilizers. However, the continuing heavy utilization of available phosphate rock resources has consumed most of the high grade rock which contains relatively small amounts of magnesium. Thus, much of the supply of phosphate rock presently available to the fertilizer industry contains so much magnesium that its removal is relatively expensive or even impractical. Accordingly, it is now considered highly desirable to develop a suitable process for producing a high-magnesium ammonium polyphosphate fertilizer solution which is stable during storage and use. Desirably, this stable, high-magnesium ammonium polyphosphate should be relatively non-corrosive to carbon steel equipment generally used during storage and utilization of the fertilizer solution.